possible weak/underactive glutes.
part of it could be a weak lower back but its probably only part of it. switch to side planks to hit the abs, obliques and low back at the same time. swap out some back squats with some atg front squats and box squats. and once and a while some goodmornings to round it out.
also, if you can find an olympic coach see if you can find one that can help with the activation of the glutes.